KNbO₃-based Ferroelectric Semiconductor Ceramics And Band-gap Enginering

Solar energy is one of the basic energy in nature,which can provide inexhaustible clean energy for the sustainable development of society.Large scale use of solar cells（devices that convert solar energy directly into electricity）are an effective way to solve the global energy crisis and deal with global warming and other problems.Perovskite ferroelectric oxide materials have a good application prospect in photovoltaic devices.The spontaneous polarization in ferroelectrics can promote the separation of carriers and make the optical voltage higher than the optical band gap,which has a great effect on the improvement of photoelectric conversion efficiency.However,due to the electronegativity difference between oxygen and transition metal atoms and the large optical band gaps（≥3 e V）,the application of Perovskite Ferroelectric oxides in photovoltaic device are seriously restricted.Based on this,KNbO3（KN）ferroelectric materials with classical perovskite structure are selected as the research object.Through ion doping,the traditional solid-phase sintering method is used to prepare ferroelectric semiconductor ceramics,so as to reduce the optical band gap and improve the photovoltaic efficiency.The XRD,SEM,Raman spectrum,optical performance,ferroelectric performance,AC impedance spectrum and photovoltaic response are studied in depth.These studies are of great significance to understand the structural transformation and band gap engineering of ferroelectric semiconductor ceramics.The main contents of this paper are as follows:（1）（1-x）KNbO3-xBaCo1/2Nb1/2O3-δ（KN–BCN;x=0.00,0.02,0.05,0.10,0.20）ferroelectric semiconductor ceramics have a single perovskite structure.The phase structure transition and the existence of ferroelectric long-range order are confirmed by Raman spectrum.XPS fitting analysis shows that the detailed formula of x=0.10 ceramic is0.90KNbO3-0.10Ba Co1/2Nb1/2O2.90.The optical band gap is 1.25 e V at x=0.10,which is61.5%lower than that of KN.In air and N2,the Ea of grain resistance（Rg）is 0.49 e V and0.42 e V,respectively.The conduction process is dominated by the first and second ionization interactions.The Ea of the grain boundary resistance（Rgb）is 1.20 e V and 1.27 e V,respectively.It can be concluded that the conduction process is caused by the double ionization oxygen vacancy.The Log I-Log U curve shows that the grains and grain boundaries follows the Ohmic behavior.The short circuit photocurrent density（Jsc）and open circuit voltage（Voc）of 0.90KN–0.10BCN after polarization is 6.68 n A cm^-2 and 0.80 V,respectively.（2）（1-x）KNbO3-xBaNi1/2Nb1/2O3-δ（KN–BNN;x=0.00,0.02,0.05,0.10,0.20）ferroelectric semiconductor ceramics are transformed from orthogonal structure to cubic structure.Raman spectrum confirms that ferroelectric long-range polarity order occurred,Residual polarizations（Pr）are decreased with the increase of BNN content,which indicats that the doping of BNN increases the leakage conductivity of ceramics.The dielectric constant and loss decrease with the increase of x.The band gap are decreased from 3.22 e V（x=0）to 0.92 e V（x=0.20）.The Jsc and Voc of x=0.10 is 1.10 n A cm^-2 and 0.77 V,respectively.（3）The XRD and Raman spectra of（1-x）KNbO3-xBaFeO3-δ（KN–BF;x=0.00,0.02,0.05,0.70,0.10）ceramics shows that the ceramics are transformed from orthorhombic phase to cubic phase,and the existence of ferroelectric long-range polar order is confirmed.The minimum optical band gap（1.82 e V）is achieved at 0.90KN-0.10BF.The area ratio of Fe³⁺and Fe⁴⁺is 2.6:2 by XPS fitting.The detailed formula of 0.90KN-0.10BF is 0.90KNbO3-0.10BaFeO2.84.In air and N2,the impedance measurement of x=0.07 shows typical p-type semiconducting behavior.The conduction mechanism of ferroelectric domain and domain wall is controlled by the first and second ionization of oxygen,respectively.The log I-log U curves show that the ferroelectric domains and domain walls of the samples shows Ohmic behavior.The Jsc and Voc of 0.93KN–0.07BF ceramics are 11.39 n A cm^-2 and 0.217 V,respectively.（4）The XRD of（1-x）KNbO3-xZnMnO3-δ（KN–ZM;x=0.00,0.01,0.03,0.05,0.07）ceramics shows that the samples have a single perovskite structure.Raman spectrum detect the existence long-range order,and with the increase of x content,the intensity of ferroelectric characteristic peaks are gradually weakened.The strains are decreased rapidly from 0.091%of x=0 to 0.006%of x=0.07.The optical band gaps are decreased from 3.22e V of x=0 to 2.27 e V of x=0.07.the Jsc and Voc of x=0.05 sample is 1.289 n A cm^-2 and 0.89V,respectively.